1. Field of the Invention
The present invention relates to the field of satellite communications and more particularly to carrier recovery networks utilized in quartenary phase shift keyed (QPSK) modems.
2. Prior Art
One of the widely used carrier recovery networks in a QPSK modem is illustrated in FIG. 1. A 70 MHz IF signal having quartenary phase shift keyed data modulation impressed thereon is applied to a multiplier 2 which multiples the IF data signal by 4 to thereby remove the data modulation. The signal is multiplied by 4 in the case of QPSK, while a multiplication factor of 2 would be employed with binary phase shift keyed (BPSK) modulation. The multiplied IF signal at 280 MHz is applied to mixer 4 where it is mixed with a 216 MHz signal to provide a nominally 64 MHz signal. The 64 MHz signal is passed through bandpass filter 6 and applied to divider 8 which divides the 64 MHz signal by 4. The output of divider 8 at 16 MHz is applied to one input of mixer 12, the other input having a 54 MHz signal applied thereto. Mixer 12 provides the sum frequency of 70 MHz as the received carrier signal.
The prior art carrier recovery network is supplied with an automatic frequency loop 10 since the carrier frequency of 70 MHz is subject to drift. The carrier recovery network locks the phase of the carrier in the AFC loop by applying input and output signals of the bandpass filter 6 to mixer 14 to provide phase information to voltage controlled crystal oscillator 16 which in turn varies the nominally 54 MHz output signal. This 54 MHz signal is applied to multiplier 18 which multiplies by 4 to deliver the 216 MHz signal to mixer 4.
Although the prior art recovery network shown in FIG. 1 works fairly well at higher S/N ratio, it introduces jitter and noise to the recovered carrier the S/N ratio drops, especially in time division multiple access (TDMA) operation, due to the intermittent nature of TDMA communications. Reducing the bandwidth of the 64 MHz filter results in a poor response, sluggishness, and increased bit error rate (BER). Furthermore, multipliers such as multipliers 2 and 18 and dividers, such as divider 8, are limiting elements in the proper functioning of carrier recovery networks used in QPSK or higher level modems. Presently available dividers are limited by the state of the art of solid state counters which do not exceed a few hundred MHz. Furthermore, each non-linear multiplication by 2 degrades the S/N ratio by at least 6 dB. The operation of digital dividers is degraded under noisy and jittery conditions, and skipping is caused in the recovered carrier.